What's a good medical alert system? Is Life Alert reputable?

If you don't mind...what is your wife's condition that you are looking for a medical alert system? In your profile it states you care for your mom.
Personal opinion as long as someone is cognizant to press a button to summon help that is great. With a diagnosis if dementia or a physical problem that would prevent activating the alert when necessary they are pretty much the same, it boils down to cost as well as what the coverage is in your area.
If the reason for the call for assistance is a slip and fall the person may or may not loose consciousness, if it is a faint and fall they will not be able to call until they are aware of what happened.
I would look for local reviews, possibly ask for references and maybe even a stop at the fire department to ask what kind of system works best in your area.

I think that people who are satisfied with their systems stick with them so they can't really tell you whether or not something else is better, it might be much more useful to ask what medic alert system people hated of thought was a rip off! The tech is evolving so rapidly - no long ago fall detection was a new expensive add on, today there are even options that work as panic buttons for seniors away from home.

One thing I disliked about Lifeline was that calls went to a distant call centre, (when we first got an alert system back in the 90's the company used a call centre that was closer to us, but that company was eventually bought out by Philips). Over the time we used the system the staff seemed to be changing to people whose first language was obviously not english. I've got nothing against immigrants but IMO when you are dealing with elderly people who may be in crisis you need not only a good command of the language but a minimal accent so you can be understood.

I’ve heard good things about Phillips Lifeline. There are different services that you are charged for which makes your monthly cost vary with the unit and features you choose.
I was a visiting nurse and used to help the families set the device up and saw how they worked. Some are motion detectors that come from a fall and an alarm is triggered.
i’ve been In the home when someone at the other end answers asking if the person is alright or if they need further help (then they call EMS).

We used Philips Lifeline. I liked that mom had to use the button to check in once a month because it got her used to using it and we all became accustomed to what to expect.

I posted about Medical Guardian. Search for that post. I researched 12 of them. I chose that one. The only problem I had was that my mother would not wear the pendant.

But I heartily recommend.